1. Field
The present disclosure generally relates to processes for making composite structures, and deals more particularly with gradient curing of composite laminate parts.
2. Background
The process of curing composite laminate parts presents several challenges where the part is contoured and/or is especially thick. In order to achieve smooth inner and outer part surfaces, both sides of the part must be tooled using tools having surfaces substantially matching the intended part shape. These tools are typically expensive to fabricate, heavy, and difficult to move. The need to tool both sides of the part layup also makes the task of vacuum bagging the part more challenging.
During curing of a contoured part, the part can warp and deform from its intended contour. For example, the part may spring out to a larger angle than intended after the curing process. Accordingly, complex modeling and calculations may be required to fabricate the curing tools with contours that differ slightly from the intended contour in order to allow the part to exactly deform to the intended contour after curing. Even where the part has flat outer surfaces, the part may change in thickness and the tools may move with respect to each other during curing. In order to compensate for this tool movement, counter-weights or other mechanisms may be used to assure that the tools move in accordance with changes in the part thickness. Deformation of the part during curing may be also occur as a result of the tools and the part thermally expanding at different rates.
Curing thick thermoset parts presents still another problem caused by overheating of the middle portion of the layup produced by the exothermic chemical reaction that occurs during resin cross-linking. This overheating is similar to thermal runaway, and is due to the rate at which the heat generated by the exothermic chemical reaction escapes to the edges or surfaces of the part layup. This overheating phenomenon may cause an undesirable change in the material characteristics or properties of the middle plies, resulting in a part that does not meet specifications and/or performance requirements.
Accordingly, there is a need for a method of curing composite laminate parts that are contoured and/or are especially thick. There is also a need for a method as described above that reduces tooling requirements, simplifies vacuum bagging, and increases manufacturing throughput.